There is growing demand for formaldehyde free and formaldehyde reduced resins for the manufacturing of composite wood and/or paper products used in buildings. These composite products are structural or non-structural panels commonly used in constructing wall, floor, roof, doors, cabinets, furniture, and architectural mouldings. The main currently used wood adhesives are formaldehyde-based resins, such as urea-formaldehyde (UF), phenol-formaldehyde (PF) and melamine-formaldehyde (MF) resins, and the composite products made of these resins are most common plywood, oriented strandboard (OSB), particleboard (PB) or medium density fiberboard (MDF) that may have various overlays and finishes.
Formaldehyde vapour is hazardous to human health; some programs such as LEED (Leadership in Energy and Environmental Design) and Greenguard allow manufacturers to earn credits for producing non-formaldehyde or low formaldehyde resin products. Such programs are becoming important elements of the marketing strategy of most manufacturers of composite products, cabinets and furniture. Consumers are increasingly asking suppliers of OSB, PB, MDF and plywood to produce formaldehyde free or low-formaldehyde containing products.
Some examples of such aldehyde free or reduced resins are known. JP2003-221571 by Umemura describes a wood adhesive composition comprising tannic acid and chitosan as essential components for wood. An acid is used to dissolve the chitosan to produce an adhesive which can be used for various types of wood products including plywood and particleboard.
JP2005-081815 by Hiromatsu discloses a plywood panel that reduce formaldehyde emissions by suppressing the amount of formaldehyde emitted from the plywood panel by using an adhesive containing chitosan that behaves as an adsorbent of formaldehyde from the heat curable resin.
Peshkova, A. et al. in Investigation of chitosan-phenolics systems as wood adhesives (Journal of Biotechnology 102:199-207, 2003) describe chitosan-phenolic systems as wood adhesives. Peshkova et al. describe that adhesive strengths of the resins tested were directly related to the viscosity of the adhesive systems and afforded lap shear strengths less than 270 psi (approximately 1862 kPa).
Although available there is still a great need for resins produced with low-formaldehyde or free of formaldehyde that have excellent performance and low cost. Developing novel types of adhesives for green composite panel production from renewable natural resource, and reducing environmental impact of VOC emissions from composite products are strategically important for next generation green building. Chitosan is an amino polysaccharide deacetylated from chitin, which is naturally occurring in large amount in shells of marine crustaceans such as crabs and shrimps and in cell wall of fungi. The chemical structure of chitosan consists of β-1,4-linked D-glucosamine residues with a number of randomly located N-acetyl-glucosamine. Chitosan is soluble in weakly acidic aqueous solutions and presents in a cationic polyelectrolyte form, which creates the possibility for interactions with negatively charged molecules. In other words, chitosan possesses an adhesive property. Chitosan has received much attention as a potential polysaccharide resource in various fields, and it has been studied extensively for medical and industrial applications.